


Sisterly Mystery

by PaddedPriestess



Category: Persona 5
Genre: Diapers, Gen, Soiling, Wetting, messing
Language: English
Status: Completed
Published: 2018-12-02
Updated: 2018-12-02
Packaged: 2019-09-05 08:42:26
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,145
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/16807273
Author URL: https://archiveofourown.org/users/PaddedPriestess/pseuds/PaddedPriestess
Summary: Makoto has access to a thumb drive with every little thing her sister has on her laptop! Maybe she can find something nice to bond with her sister over… Or maybe she’ll find a secret she could have never anticipated.





	Sisterly Mystery

“Hey, Futaba, mind if I ask you a favor?” The redheaded NEET squeaked and jumped in her chair, quickly spinning around to figure out why somebody was still in her room. “What are you still doing here!? Everybody left a couple minutes ago! Shouldn’t you be gone too?” Makoto shook her head at the smaller girl in front of her. “Yes, well, as I said, there’s a favor I have to ask.”

Futaba sighed a bit, and then nodded her head. “Sure. What do you need?” Makoto’s eyes darted around her impressive desktop setup, seemingly scanning for something in particular. “Remember when we had to copy my sister’s hard drive to a USB? I wanted to ask if you had deleted everything off of it or not.”

The tiny NEET shook her head, scrounging around the mess that was her desk, before procuring a tiny thumb drive. “This?” The Student Council President’s eyes lit up at the sight. “Yes! That. Would you mind if I took it? There were some things I wanted to look over.” Futaba shrugged, and then tossed the little device over. “Sure. But don’t you dare lose it! That’s my 2nd largest thumb drive.” She warned.

Makoto caught the USB in hand, and then nodded quickly. “Don’t worry; I’ll take good care of it. Thanks again!” She waved at Futaba before finally evacuating the small room, leaving the NEET alone with her beloved computer.

With USB clutched in hand, Makoto quickly made her way back home. Her sister wouldn’t be home, and so she’d have the time to complete her ‘mission’. It wasn’t too much of one, but she had to know just a bit more about her sister. Sure, it was a work laptop, but she had to have something on there that could help learn about her, right? If Sae took the time to write her personal thoughts like some electronic diary, then maybe Makoto could either help alleviate some of her stress, or attempt make herself into a better sister. That was a good reason to look through her personal data, yeah!

After a fairly noneventful walk home, the younger Niijima walked into her house, wasting no time in dropping her school bag by the side of the couch before rushing over to her own laptop. Her sister didn’t normally spend too much time at home, but she would still show up around dinner, so chances are now would be the best time to get the initial scouring done.

She started off simple; making sure to get what she figured was the boring stuff out of the way. A couple folders designated only by numbers, which when clicked on, only ended up being as she expected. Work files, general information, that kind of stuff. Not the best start for her snooping.

Next were the folders with letters rather than numbers. They were probably the exact same, just by different organization, but felt she might as well go ahead and check them all anyway. She had to be precise. Even if Sae wasn’t, considering some letters were entirely missing. It skipped from A, to C, to D, to F, and so on. How unusual.

Regardless, Makoto went to click folder A, then a thought occurred to her. If her sister wasn’t trying to hide anything, maybe D would actually stand for her diary? She’d go back to A afterwards, so a little detour wouldn’t help. She hovered her mouse over it, and clicked to enter the album.

Nothing seemed too out of the ordinary here. The only strange thing was that it was organized with photos, not the usual text documents. The first ones were just of Sae in her usual business attire, staring into the camera, probably to check the angle and make sure it worked. After that, it was just her in various poses, front and back, camera angled slightly low so that the focus was on her midsection.

It didn’t stay that way for long. Makoto’s eyes darted to a picture with a strange color contrast. Sae always wore dark colors, so her in white was unusual, to say the least. Of course, realizing what she was looking at made her blush. Her sister had her pants pulled down, white panties on display for the camera. Her sister was showing off! What was she thinking? But it only got weirder as Makoto opened the picture to closer examine it.

Those weren’t panties at all! Panties weren’t so large, and thick! That… That was a diaper her sister was wearing! It couldn’t be anything else. But why? Why was her sister wearing a diaper? Did she do that normally and Makoto just didn’t notice? Or did she only do it when home alone? She had to find something!

She closed the picture, and then turned to skimming through the pics instead. More poses, different angles, and what appeared to be a maid in some shots, though her face was censored out for what Makoto presumed were legal reasons. Searching through the rest of the pictures didn’t get her any closer to finding out just why her sister was wearing diapers.

Though… Judging from the way the later pics looked, it appeared Sae also actively used the diapers she was wearing. There was a huge brown bulge in the back whenever she had her butt visible to the camera, and when the front was visible, it was obviously sagging further down than it had been earlier, not even to mention the yellow tinting across the whole front and middle.

That was enough for Makoto, who quickly just shut the laptop and removed the thumb drive. Now she knew more about her sister than she probably wanted to. But, there was no removing that knowledge now. She continued to just mull it over in her head. It was strange, very strange, and something she never would’ve expected. Though, at the very least, it wasn’t anything harmful, or incriminating, or otherwise potentially volatile for them.

But now there was this strange feeling in her chest. She just… She had to confirm it with her own eyes. Not to the point of ambushing Sae and pulling her pants down or anything, but she still had to see it with her own eyes and not on a laptop. Setting her things back into her bag, she looked around paranoid, as if expecting her sister to arrive home at this exact moment.

After a couple moments of silence, she decided to follow her impulse and head off to her older sister’s room. Just out of worry, she made sure there was no way she could be caught; adjusting the door’s handle ever so slightly, opening it slowly, and making sure she didn’t wear anything that might fall off, like a necklace. When she confirmed that she was indeed stealthy, she finally entered the room fully, leaving the door open as an alarm of sorts.

Now for the fun part; actually finding the package of diapers that Sae was hiding. If she hadn’t already used all the ones she had, of course. Normally she would anticipate her sister would hide them well, but considering Makoto never had reason to come in here, she might’ve felt safe enough to just put them somewhere inconspicuous. Or, for a third option, maybe she put it in plain sight because she wanted to get caught. Makoto couldn’t begin to fathom just how far her sister’s weird fantasies must go.

She walked into the middle of the room, glancing around for what might work. First, under Sae’s bed… No, it was entirely clean. Nothing there at all, unsurprisingly. After that, she headed over to the closet, and swung it open. She sifted through the numerous business suits, the occasional formal dress, her leisure wear, and… Ah! There they were. Just sitting on the floor. Oh Sae…

Makoto bent down to her knees, afraid to move the package out of its current place. It was already open, judging by the huge tear in the top, and a couple of diapers were missing. At the very least, enough that it wasn’t well organized inside, some of them toppled over others. She couldn’t even count how many there were because of that.

Actually… That wasn’t a bad thing. She reached over and tentatively put her hand inside, feeling the soft plastic for herself. This is what her sister liked to wear… She almost couldn’t believe it. Taking just one wouldn’t hurt, would it? She grabbed one from the bottom, sure her sister wouldn’t notice. Now she slowly drew it out, trying her best to not upset too much of the packaging. Finally, she pulled it out, putting the diaper her side and trying to make the package seem like it hadn’t been touched by even a light breeze.

Once she’d done what she could, she picked up the diaper again and stood up, looking it over closely. Sae liked to wear big, cushy things like these, hmm? She drew her fingers across, eliciting a quiet crinkle, and then squeezed it. Yeah, it was pretty thick. Sae probably never needed a bathroom break so long as she was in these.

Then a more important thought crossed her mind. In those pictures, while embarrassed and blushing, her sister was noticeably happy. Not just the façade she put on, or any attempt to smile awkwardly for the camera. She actually appeared to be enjoying herself. In a couple, at least. Maybe this could be the key to getting closer to her sister?

Her heart started to beat faster as she thought it over. If she wore diapers like Sae, could that make them become closer? A dirty little secret to share, something only the 2 Niijima sisters could bond over. It was a heartwarming thought, but her own happiness came first. She’d have to try it on and see if she actually liked it. Maybe a love for diapers ran in the family?

She tucked the diaper under her armpit, and then closed the closet door, giving a once-over of the room to make sure she wouldn’t leave behind any evidence. It’d be embarrassing if a phantom thief got caught stealing diapers from her older sister. When the coast appeared clear, she walked out of Sae’s room, closing the door behind her and then heading off to her own room.

Once inside, she rapidly displaced her lower clothes, lifting her skirt and pulling stockings and panties down in one quick maneuver. Then, she unfurled the thick padding and set it lightly on the ground. She’d never babysat before, or even seen a diaper since she herself was being potty trained. She really had no idea what she was doing, but it would be a fun learning experience… Right? If everything went well, maybe Sae could help her out in the future.

She gently sat on the open diaper, wiggling her hips to try and place herself just below the center. She was a smart girl; she could figure out that much! Stretching her legs as far as she could with her clothes still on, she carefully brought the front up between her thighs, making sure it was even with the back and trying to get as comfortable as possible.

She dropped her legs back down, still holding them apart, and then started trying to work the tapes on. One… Two… Three and four. She pulled them all on as tightly as her wide hips and large rear would allow, before looking down at herself. Her first feeling in her new diaper, and it was a huge sense of just how silly it was. A high school girl, president of the student council, wearing an adult diaper. She’d probably be more embarrassed if the memories of her sister wearing a soiled diaper weren’t burned into her memory.

Now came the moment of truth. She slowly pulled her panties and stockings back up over her legs, then over the thick diaper. They still fit well, and it all made for a pretty snug getup. Now she stood up, listening to the quiet crinkling of plastic under fabric as she did. It wasn’t too bad. A bit thick, and her legs weren’t able of sticking as close to themselves as she expected, but it wasn’t anything overtly weird.

She took a couple tentative steps, gauging her gait like this. She could walk normally for the most part, so it made sense how Sae was able to hide these. All she couldn’t understand was how she’d be able to hide any foul smells or leaks, though knowing her sister, she probably had contingency plans for taking care of issues like that.

She waddled out to the living room, going right back over to where her laptop was, and sitting down quietly. Except for the feeling of silliness and a bit of embarrassment, it wasn’t half bad. Her butt felt a lot more comfortable now, and there was a strange feeling of… She couldn’t quite put her finger on it. Security? It did feel kind of relieving to have on something so snug and warm and cushy.

But, the final part of the test was seeing how long she could last like this. If it started to get uncomfortable or something, then she wouldn’t be able to go through with all of this after all… The thought honestly saddened her. She wanted to have something like this to share with Sae, especially with everything that’s been working to drive a wedge between them. She had to like diapers! Even if she ended up walking around in her own pee and poop like her older sister!

Unfortunately for her resolve, she didn’t feel any need to use the bathroom. So, instead, she pulled her laptop back out and started reading general articles on the internet. Anything to help make this seem like a more usual occurrence. If she could make it so she barely realized she was wearing a diaper, then it’d be a success!

She sat there for an hour or two, mindlessly browsing the internet for distractions, when the sound of the door opening quickly redirected her attention and almost set her into a panic. “I’m home.” Came the voice of her older sister. Sae was back! How would she get across the fact she was wearing one of her diapers? Later, at dinner maybe… She had to think of the perfect way to introduce it.

Sae walked over to the table, looking down at Makoto, as if just to reaffirm her presence at the house. “Ah, welcome back, big sister. How was work?” She asked. General questions, trying to keep herself distracted from accidentally blurting something incriminating out. But even then, she couldn’t help making her eyes wander to the prosecutor’s hips. Was she wearing one now? It was hard to tell.

“Same old, same old. No new leads, and in fact one of the witnesses of the most recent case refused to have any word with us. We’re at a minor deadlock.” She sighed. Honestly, it would be understandable if she did this for stress relief. Makoto knew she’d need that a lot. “But, I don’t want to say too much on the matter. How was school?”

“Nothing really interesting here either. Even the student council duties are winding down.” She responded. Sae nodded in return. “Well, average for both of us then. I’ll come out and make dinner in a bit, alright? I’m gonna go relax for a bit.”

“Sure thing! Take your time.” Makoto replied, watching Sae walk off, her eyes especially focused on her sister’s ass. She still couldn’t tell, although considering she was going to relax, chances are she was about to put one on if she hadn’t already. Good. If they were both diapered, then maybe it’d be easier for her to break the news.

At the very least, she kept her cool. As nervous as she was, she didn’t get caught. Although, the thought of it did bring to attention the fact that her bladder was full. That jolt made her realize, though thankfully, she didn’t wet her pants. She’d have to be quite the infant to manage that just because her sister scared her.

She readjusted her seat, and went back to perusing the laptop. Now that she’d become more acclimated to her diaper, maybe she could go find advice on how best to tell her sister. There wasn’t room for any mistakes when it came to something this important, so she had to get it right the first time! She pulled up the browser, then looked up over it when she noticed Sae walk back out. “Oh, is it time for dinner al-“

“Did you take one of my diapers?” Sae shot the dreaded question at her younger sister.

Makoto froze. Just like that, everything she was planning had been ruined! She couldn’t even think of a proper excuse due to the way Sae so pointedly asked the question. “Wh-what? I…” She stuttered out, scared. How did her sister find out!? She’d been so careful! Just stay calm, and think of an answer, she thought to herself.

Sae folded her arms and stared at Makoto in her usual, stern way, like she was working on prosecuting one of her cases. That glare sent shivers up her spine, and she started shaking in her seat a bit. As if that wasn’t bad enough, she felt her floodgates open unwillingly, and she started wetting her diaper right in front of her sister.

“I… Yes, I did…” She couldn’t lie to Sae. Not even about something this embarrassing. Especially now that she was currently using her padding like a potty, something her sister probably did on average. It was just too unfortunate she couldn’t gauge how it felt, considering she was staring down her probable punishment now.

Sae sighed, and closed her eyes for a second. Makoto couldn’t help but worry about what she was going to say next, and it wasn’t helping that she was still in the process of wetting herself. The silence was eerie, and she could almost swear the sound of her peeing herself was audible to Sae. Though she absolutely hoped it was not.

“Why?” Her sister finally spoke, right as Makoto finished wetting her diaper. Her groin was covered in warmth, and her nervousness made her rub her thighs together unconsciously, making it squish quietly under her skirt. She hadn’t anticipated this at all. She could still barely comprehend what was going on.

“I… Uh…” Was all she could stutter out for the moment. She had to think quickly. Exposing that she found out because of Sae’s pictures would expose that she was the one who tampered with Sae’s laptop to begin with, and she couldn’t do that. Whether or not it might be minor, there was still an ever present chance it could be linked back to the Phantom Thieves, and Makoto couldn’t let that happen.

“I thought one of my skirts got mixed up in your stuff, so I went to your closet to go check. Then, I noticed your diapers. I figured they were for you, so I thought maybe you’d like it if I also wore diapers…” She half-lied, half-admitted, blushing heavily and kicking her legs under the table.

To her relief, Sae’s expression softened. She was less agitated, less intimidating. If anything, she almost looked relieved herself. With a soft sigh, she spoke. “That’s all? Such a strange little sibling I have.” A remark meant for herself and for Makoto, considering the state of both girls. “Well, will you stand up and let me see?” She asked.

Makoto slowly nodded and got up to her feet, shimmying out from the seat and into the open, still blushing as she felt the wet warmth squish around with her. Sae approached, reaching down and casually lifting Makoto’s skirt up with one hand. “So you really did…” Her free hand reached down and cupped the front of Makoto’s diaper through the fabric of her tights. “This… Was my fault, wasn’t it?” She asked, figuring she scared her sister so hard she wet herself.

“N-no, it’s not! If anything, I wanted to try it, just to see how it felt…” Another half-lie, half-admittance. She did want to know how it felt, but in a much less terrifying way. Regardless, her older sister let go of her outfit, hiding her wet diaper again. Being treated this way was still a very foreign feeling to her, but even if she wasn’t being 100% honest, she was still feeling less frightened now that Sae’s face had softened.

“Well, I suppose I owe you as much then.” She whispered, causing Makoto to tilt her head.

“Huh?” She had no idea what Sae meant by that. Instead of answering, her sister instead pulled down her own pants, revealing her own crinkly, white diaper. Or at least, white in some areas, as the front of hers was stained an obvious yellow as well. She must’ve worn that to work! And used it heavily… But her observation was interrupted as the padded prosecutor suddenly turned around, sticking her butt right near Makoto’s face.

“Uh oh… I think I’m gonna…” Sae said. “Uuugh… Hnnng…” She started to groan out, and Makoto realized exactly what she was doing. She was taking a dump in her diaper! The younger sister’s eyes widened at the sight. The back of Sae’s already wet diaper started to bulge out slightly, and in the close proximity, Makoto could even hear the squelching, crackling noises coming from her sister’s butt.

“I’m pooping…” Sae added on, before giving another hard grunt, the lump in her diaper growing even fatter and sagging as her mess piled in. For as lithe as her figure was, she sure could poop a lot. By now, the seat of her diaper was at least triple the size it was before, the huge lumps making quite the indentation. It was just like those pictures she took… Did she ever use the toilet?

With a loud sigh of relief, Sae pinched off the last of her mess, and took a couple breaths. “I pooped my diaper…” She announced to Makoto, as if it wasn’t already completely obvious. After all, even if the sight of it wasn’t enough, the sounds and especially the smell would’ve given it away.

Tentatively, as if she was afraid it would bite, Makoto stuck her hand out to her elder sister’s stinky diaper, pressing her hand into the seat and squishing it ever so lightly. “Makoto… How big is it? Did I make a huge poopy?” She asked.

Makoto could only really nod, still in a weird haze at the scenario she was stuck in right now. This all had actually happened! What was even going on anymore? “Y-yeah, you really filled it…” She pushed the mess in a bit more, getting a little squeak out of Sae. Her older sister, known for her stoicism and intimidating personality, just squeaked like a cute little girl.

“Well, I guess it’s to be expected at this point…” She sighed, finally straightening herself back up before pulling her pants back up over her diaper. Then she turned to face Makoto, with what could only be described as an earnest smile on her face. “Us Niijima sisters just don’t seem quite adept at potty training, do we?” She asked with a chuckle.

Her smile was infectious as well, Makoto also grinning a bit shyly. It was all still new to her, but in the end, she was right about it all. She and her sister could get along through this! Wearing and using diapers… Strange, but she wasn’t complaining. It was exactly what she asked for.

“I guess not. Maybe we’re just destined to wear diapers?” She responded, Sae folding her arms again, but in a more lax way.

“Well, maybe you’d be better suited to being the big sister sometimes then.” She teased, patting Makoto on the head.

“Looks like I’ll have no choice, seeing what you do in your diapers…” She taunted back, both sisters now laughing together.

“Say… How about you sleep with me tonight? For old time’s sake.” The elder sister asked, gaining a confused head tilt in return. “My bed’s big enough for both of us, so I’m sure even with our diapers, we can both fit.” Makoto snickered again, nodding along with the idea.

“I guess it wouldn’t hurt tonight… Just be warned, I’ve started wetting the bed again.” She said.

“Started? I never stopped.” Sae retorted. Makoto sighed in return, albeit playfully.

“Then maybe we should change you before bed? Your poor diaper probably won’t be able to hold on for very long.”

Sae pressed her hand against the back of her own diaper, feeling it for herself. “Hmm… We’ll see~”


End file.
